TCP 3-way handshake - multiple SYNs

Question

Suppose host H1 is looking to make a TCP connection with H2 and sending a SYN for the 1st step of 3-way handshake. H1 then isn't hearing back in timeout and is sending a second SYN to H2.

Then, H1 is hearing back on both of these SYNs. What does H1 do in this case-- which one of these two does it ACK as the 3rd step of handshake, the first SYN+ACK from H2 or the second? The two SYN+ACK have 2 different Initial Sequence Numbers (ISNs) of H2. They also have different ACKs-- H1 sent 2 different ISNs in its two SYNs.

From another point - in such a case, what does H2 do? So - H2 received an SYN and sent SYN+ACK in response to it. After a while, H2 received another SYN+ACK from H1. What does H2 do in this case:

send SYN+ACK to the second SYN as well? in this case, how does H2 know which SYN (the ISN of H1) is the one for this connection?

or

ignore the second SYN. But then, H2 timed out as well-- it didn't hear back any since it send the previous SYN+ACK.

How is this handled in TCP?

TIA

Note: I've seen What if a TCP handshake segment is lost?, TCP: SYN request receives SYN response instead of SYN-ACK, tcpip 3-way handshake and TCP handshake reliability along with some other useful discussions.

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EDIT:

The answer seems to lie in the RST flag-- resetting whenever in doubt of the sequence#s. See Figure 9 of RFC793.

However, the Q still stands in the light of the comment below: when does H1 ("TCP A" in Figure 9) retry the connection with a new ISN of its-- how long after and under which circumstances?

I'm voting to close this question as off-topic because it's not programming related — Alnitak
The sequence number for a resubmitted SYN is the same as in the original SYN. What made you think this is not the case? — Steffen Ullrich
@SteffenUllrich - the ISN is read from a counter ticking every "4 milliseconds" if I remember correctly. I assumed going by that clock in the second attempt. have you a reference handy to this? — Roam
@Roam: missing packets (which includes SYN) are retransmitted which means that exactly the same packet is sent again. The relevant standard is RFC 793. — Steffen Ullrich
@SteffenUllrich - how then is the situation in Figure 9 of RFC793 coming up? See my edit in the Q-- portion below the line comment. — Roam

Joni Joni · Accepted Answer · 2016-02-22T23:44:07

The situation in figure 9 happens when the SYN from a previous connection attempt arrives after the sender has already given up on that connection. Host B sends an ACK for this extra SYN, and Host A sees that the sequence number of the ACK is wrong. Then Host A aborts the connection by sending an RST.

TCP 3-way handshake - multiple SYNs

2 Answers